In nonlinear optics, the basic role of any material medium is to act as a coupler between incoming light waves that would not interact otherwise, i.e. in vacuum or in the case of weak fields.
Such interaction gives rise to radiation of new light fields whose properties, such as frequency, amplitude and phase, may differ from the incoming fields. The relation, however, between in- and out-going light fields properties is in general not linear. Furthermore, the light field properties themselves, such as frequency, amplitude and phase, are interdepend and cannot be controlled independently.
On a microscopic level, physical limitations of nonlinear optical interaction arise from the dispersion properties of matter, such as, for instance, the frequency dependent speed of light in dielectric media or the phonon lifetime in solids. For many nonlinear processes to be efficient, it is a necessary condition that all interacting light pulses are phase matched, i.e. that they have the same phase velocity and the same group velocity. Satisfaction of the latter becomes more and more difficult the shorter the pulses get or the broader their spectra are, respectively.
Currently in the art, ways to minimize phase matching problems include: i) reducing the length of the dispersive medium, which typically reduces the efficiency of the process, ii) changing the angle of interaction, iii) modifying the pulse fronts of interacting beams, and iv) using periodically poled crystals. All these methods take place in the time domain.
Another fundamental limit for nonlinear interaction is given by the damage threshold of the material medium employed. To overcome this problem and avoid material damage, i) the beam size may be increased or ii) the pulses may be stretched in time, to lower the peak intensity. Enlarging beam sizes is limited by the availability of large aperture optics such as for instance nonlinear laser crystals.
A number of publications deal with performing light amplification in a spatially dispersed plane (SDP) [1-5]. They all involve real level pumped gain media, i.e. stimulated emission from a population inverted medium, to amplify ultrashort pulses that have been previously generated by other means. In other words, in these cases the SDP is seeded with photons and the properties, i.e. frequency and phase of these photons, are preserved, the interaction only causing an increase in their number. These amplification systems are said to be linear.
There is still a need in the art for a method and a system for modifying photon properties through nonlinear optical processes.